Chaplains with jump packs are truly amazing. Chaplains are all about the assault, and jump pack when attached to a assault squad is the cheapest way for a quick assault, land raiders being the other far more expensive option, and bikes lacking a decent assault squad to go with. On his own most of the rerolls are wasted, as is the fearless so that's not good. On foot you're looking at 3rd at least turn assault, and that is poor. In transports they are likely to end up entangled, and can't assault on turn of getting out so 3+ turn assault. Jump packs are the best option, and I often take them.